It is a known technique in the prior art to adjust the distance between the tip of the push lever that abuts on an object into which a nail is driven (“the nailed object” hereafter) and the tip of the driver blade at the lower dead center from which a nail is ejected, namely the distance between the nailed object and driver blade in order to drive a nail into the nailed object in the manner that the head of the nail driven by the nailing tool is flush with the surface of the nailed object. For example, the driving machine disclosed in Patent Literature 1 below comprises a driving depth adjusting device in which the part of the push lever that makes contact with the driving machine body is threaded in the body using a screw. The operator shifts the knob in which the screw is housed in the axial direction of the screw to adjust the upper dead center of the push lever. In this way, the distance between the tip of the push lever and the tip of the driver blade at the lower dead center is adjusted.    Patent Literature 1: Unexamined Japanese Patent Application KOKAI Publication No. 2003-136429
The pressure of the compressed air supplied to the nailing machine is generally set for a relatively wide range of values to cover a wide range of applications. When the adjusting device described in the above Patent Literature 1 is used for driving a short nail, the operator adjusts the position of the upper dead center of the push lever to increase the relative distance between the lower dead center of the driver blade tip and the push lever tip (the nailed object) in order to prevent the nail from being driven excessively deep. When the operator drives a nail into the nailed object in this state, the piston bumper absorbs excess energy after the nail is driven. In this way, the piston bumper receives a large load and has a short durability life. Consequently, a problem is that the nailing machine has short durability life.